Apparatus for uniformly coating a substrate

ABSTRACT

A method of and an apparatus for coating a substrate with a polymer solution to produce a film of uniform thickness, includes mounting the substrate inside an enclosed housing and passing a control gas, which may be a solvent vapor-bearing gas into the housing through an inlet. The polymer solution is deposited onto the surface of the substrate in the housing and the substrate is then spun. The control gas and any solvent vapour and particulate contaminants suspended in the control gas are exhausted from the housing through an outlet and the solvent vapor concentration is controlled by controlling the temperature of the housing and the solvent from which the solvent vapor-bearing gas is produced. Instead the concentration can be controlled by mixing gases having different solvent concentrations. The humidity of the gas may also be controlled.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 08/566,227 filedDec. 1, 1995 which application is now U.S. Pat. No. 5,670,210 which is acontinuation-in-part of Ser. No. 08/330,045, filed Oct. 27, 1994, nowabandoned.

BACKGROUND OF THE INVENTION

The manufacture of integrated circuits involves the transfer ofgeometric shapes on a mask to the surface of a semiconductor wafer.Thereafter the semiconductor wafer corresponding to the geometric shapesor corresponding to the areas between the geometric shapes is etchedaway. The transfer of the shapes from the mask to the semiconductorwafer typically involves a lithographic process. This includes applyinga photosensitive pre-polymer solution to the semiconductor wafer. Thesolvent in the pre-polymer solution is removed by evaporation, and theresulting polymer film is then baked. The film is exposed to radiation,for example ultraviolet light, through a photomask supporting thedesired geometric patterns. The images in the photosensitive materialare then developed by soaking the wafer in a developing solution. Theexposed or unexposed areas are removed in the developing process,depending on the nature of the photosensitive material. Thereafter thewafer is placed in an etching solution which etches away the areas notprotected by the photosensitive material. Due to their resistance to theetching process, the photosensitive materials are also known asphotoresists. These may for instance be sensitive to ultraviolet light,electron beams, x-rays, or ion beams.

The high cost of the photoresist pre-polymer solutions makes itdesirable to devise methods of improving the efficiency of the coatingprocess so as to minimize the polymer solution's consumption.Furthermore, thickness uniformity of the photoresist layer is animportant criterion in the manufacture of integrated circuits. Itensures satisfactory reproduction of the geometric patterns on thesemiconductor wafer.

The solvent in the photoresist tends to evaporate during application,increasing the viscosity of the polymer solution and inhibiting theleveling of the resulting film. This produces thicknessnon-uniformities. It is therefore desirable to be able to control therate of evaporation of solvent from the polymer solution.

Environmental humidity is one of the factors affecting the thickness ofthe photoresist layer. Typically photoresist coating uniformity of theorder of 15 to 20 angstroms within a wafer and 20 to 25 angstroms fromone wafer to the next and from batch to batch and from day to day isrequired. This is less than the effect of a 1% difference in relativehumidity. Furthermore, in commonly used positive photoresists employingphotosensitive diazoquinone compounds, some water content is required toreact with products of the photolytic reaction to form requiredwater-soluble carboxylic acids.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of and an apparatusfor improving the thickness uniformity of a polymer solution applied toa surface of a substrate such as a semiconductor wafer.

It is a further object of the invention to improve the consumption ofpolymer solutions such as photoresist pre-polymer solutions used in thecoating of substrates.

According to the invention there is provided a method of coating asurface of a substrate with a polymer solution which includes mountingthe substrate inside an enclosed housing, passing a control gas into thehousing through an inlet, depositing the polymer solution onto thesurface of the substrate in the housing, spinning the substrate, andexhausting the control gas and any solvent vapour and particulatecontaminants suspended in the control gas from the housing through anoutlet.

The control gas can be a solvent vapour-bearing gas or a solvent-freegas.

The step of exhausting the control gas and any solvent vapour andcontaminants can be performed before, during, or after the depositionstep.

The solvent vapor-bearing gas is typically produced by bubbling a gasthrough a solvent and can include the step of controlling the solventvapor concentration of the solvent vapor-bearing gas by controlling thetemperature of the solvent. The solvent vapor concentration of thesolvent vapor-bearing gas can also be controlled by controlling thetemperature of the housing or by mixing the solvent vapor-bearing gaswith a second gas having a different solvent vapor concentration.

The solvent vapor-bearing gas typically comprises air or an inert gassuch as nitrogen.

The control gas can be passed into the housing through a showerheaddispenser located directly above the substrate to ensure continuous,controlled, laminar gas flow over the wafer.

The polymer solution can contain a photoresist polymer, for example, adeep-ultraviolet photoresist polymer.

The method can include the step of passing temperature controlledsolvent-free, dry, filtered gas over the coated substrate. The methodcan also include the step of passing solvent-free, humid gas over thecoated substrate; the humidity of the humid gas can be controlled tohave the relative humidity required by the polymer solution. Therelative humidity lies typically in the range of 40% to 45%. Thetemperature of the humid gas can also be controlled by means of atemperature and humidity controller.

The step of mounting the substrate in the housing can include securingthe substrate to a rotatable chuck, for example, by establishing avacuum between the substrate and the chuck.

The substrate typically comprises a semiconductor wafer and the solutecontent in the polymer solution is typically 10% to 50% by weight.

Further, according to the invention there is provided a coatingapparatus for coating a surface of a substrate with a polymer solutionwhich includes an enclosed housing, a rotatable chuck mounted in thehousing for supporting the substrate, a depositing means for depositingthe polymer solution onto the surface of the substrate in the housing, acontrol gas supply means connected in flow communication with thehousing for supplying a control gas to the housing, and an exhaust meansconnected to the housing for exhausting the control gas and any solventvapour and particulate contaminants from the housing.

The depositing means can include a dispensing head means mounted abovethe chuck for dispensing a stream of the polymer solution onto thesurface of the substrate, the dispensing head means being moveablerelative to the substrate. If the substrate has a substantially circularshape, the dispensing head means is typically moveable substantiallyradially across the surface of the substrate. The depositing means caninstead comprise a film extruding means having an extrusion head mountedabove the chuck for dispensing a stream of the polymer solution onto thesurface of the substrate. In this case, if the substrate issubstantially circular in shape, the extrusion head is typically mountedabove the chuck for dispensing a radially extending stream of thepolymer solution onto the surface of the substrate.

The rotatable chuck is typically connected to a variable speed motor,and the coating apparatus can include a controlling means forcontrolling the speed of the variable speed motor.

The housing can have an upstream side and a downstream side; the solventvapor-bearing gas supply means can include an inlet to the housingmounted at the upstream side of the housing, and the exhaust means caninclude an outlet mounted at the downstream side of the housing. Thecontrol gas supply means can include conduits connected in flowcommunication with the housing, and electrically-controlled valves in atleast one of the conduits for controlling the rate of control gasflowing into the housing and the composition of the control gas. Theexhaust means can also include a valve means for controlling theexhaustion of the gas and any contaminants from the housing. The solventvapor-bearing gas supply means can include a clean, dry, filtered gassource and a bubbler connected in flow communication with the housing.

The coating apparatus can, further, include a temperature and humiditycontrolled gas source connected in flow communication with the housing.The temperature and humidity controlled gas source can include atemperature control means and humidity control means for controlling thetemperature and humidity of the gas supplied by the temperature andhumidity controlled gas source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic sectional side view of one embodiment of a spincoating arrangement in accordance with the invention; and

FIG. 2 shows a schematic sectional side view of another embodiment of aspin coating arrangement in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention pertains to a method of improving the thicknessuniformity of coatings formed when depositing solutions on substrates.It pertains further to a method of reducing the wastage of suchsolutions. In particular, the method will be described with reference tosemiconductor wafers used in the manufacture of integrated circuits andthe application of photoresist pre-polymer solutions to a surface of asemiconductor wafer. It will be appreciated that films or coatings usedin integrated circuit manufacture are not limited to photoresist layersand could, for example, include materials such as organic planarizationfilms, anti-reflection films, siloxane spin-on-glass films, polyimidefilms, and polyimide siloxane films.

As mentioned above the solute content in these materials prior to thecoating process typically ranges from 10% to 50% by weight.

As will become clearer from the discussion which follows below it isdesirable to conduct the coating process of the semiconductor wafer in ahousing or chamber which allows the atmosphere in the chamber to be atleast partially saturated with solvent molecules. This has the advantageof improving the cast film wetability by establishing a monolayercoating of solvent on the surfaces of the substrate. Furthermore bycontrolling the concentration of solvent in the gas within the chamber,thickness uniformity of the polymer solution film on the substrate canbe improved. This will be the case whether spin-cast films, spray-coatedfilms or any other similar coating method is adopted.

In a spin-cast method, a solution is applied to the substrate while thesubstrate is either stationary, moving linearly, or rotating. Thereafterthe substrate is spun to spread the solution over the surface of thesubstrate. After the solution has spread over the substrate surface, thesolvent in the solution is removed by evaporation leaving a layer ofsolute on the surface of the substrate. As substrate sizes are increasedor when attempts are made to reduce costs by minimizing the amount offluid applied to the surface of the substrate, a non-uniform thicknessin the solute layer on the substrate typically results. This is, inpart, a result of the difference in tangential velocity between theperiphery and the center of the substrate. The non-uniform air flowalong the periphery causes non-uniform evaporation of solvent, therebyproducing coating thickness non-uniformities. Since the largersubstrates require higher spin speeds to obtain uniformity near thecenter, spirals and striations result near the periphery of thesubstrate due to non-uniform interaction with the air in contact withthe solution near the periphery. These features are known as Ekmanspirals.

Problems are also encountered when insufficient coating solution isused. When attempts are made to reduce costs by minimizing the amount ofthe coating solution applied to the surface of the substrate duringspin-casting, non-uniformities are created due to the low solventvolume. Evaporation of the solvent during the coating process results invarious defects and irregularities. Similarly, in spray-coated films,solvent tends to evaporate during the application, thus increasingviscosity and inhibiting the levelling of the resulting film, againresulting in thickness non-uniformities.

As mentioned above, certain photoresists require some water content toreact with products of the photolytic reaction. For these reasons, it isdesirable to be able to control the humidity of the air in the chamber.

The invention will now be described in greater detail with respect toembodiments making use of a spin-coating process. In these embodimentsthe substrate is a semiconductor wafer and the solution applied to thesemiconductor wafer is a photoresist pre-polymer solution.

FIG. 1 shows an embodiment of a spin-coating arrangement 10 used inaccordance with the method of the invention. The arrangement 10 includesa rotatable support chuck 12 mounted in an enclosed housing 14. Thechuck 12 extends into an axle 16 which passes through an opening 18 inthe housing 14. The housing 14 includes an input in the form of ashower-head-like dispenser 20. This allows control gas, comprising a gasand a certain concentration of solvent to be passed into the housing 14.The control gas can be a solvent-free gas or a solvent-bearing gas andcan include air or an inert gas such as nitrogen. The dispenser 20 ismounted directly above the substrate that is mounted on the chuck 12. Aninput conduit 24 extends into the shower-head-like dispenser 20 at itsone end. A temperature and humidity controlled gas source (not shown)supplying temperature and humidity controlled air or nitrogen isconnected to the conduit 24 by means of a conduit 26. A second conduit28 extends from a clean, dry, filtered gas source into a bubbler 30. Thebubbler 30 is housed in a solvent tank 32 containing a solvent 34. Theclean, dry, filtered gas, which typically comprises air or nitrogen, ispassed through the bubbler 30 to form a solvent-bearing gas which ischannelled to the conduit 24 by means of a conduit 36. A valve 38 ismounted in the conduit 26 and a valve 40 is mounted in the conduit 36.The valves 38, 40 allow either one or both the temperature and humiditycontrolled gas and the solvent-bearing gas to be channelled to thehousing 14. The valves 38, 40 are typically electrically-controlledvalves for automated control of gas flow rates and compositions. Thetemperature of the solvent-bearing gas supplied by the bubbler 30 iscontrolled by means of heating/cooling coils which control thetemperature of the gas supplied by the conduit 28 or of the solution 34or both. Normally heat must be supplied to the solvent 34 to compensatefor heat loss due to evaporation. The temperature and humidity of thetemperature and humidity controlled gas are also controlled using aspecial temperature and humidity controller comprising a refrigerationunit, a boiler, and a temperature and humidity sensor. Instead thetemperature and humidity of the temperature and humidity controlled gascan be controlled by means of a temperature and humidity controlleremploying a bubbler arrangement. In a preferred embodiment the conduit26 is supplied by two branch conduits (not shown). This allowsconnection to either a bubbler or a humidity controlled source. Thebubbler is typically mounted in an outer housing containing theremainder of the arrangement 10. In contrast the special humiditycontroller proposed above comprises a separate structure. When humid airis supplied the relative humidity is maintained at a level required bythe polymer solution: typically between 40% and 45%. Clearly thehumidity can be kept at zero in appropriate circumstances.

The arrangement 10 further includes a dispensing head 42 for dripping asolution (in this case a photoresist pre-polymer solution) onto a wafer44 mounted on the chuck 12.

The bottom of the housing 14 defines an annular channel 46 having anexhaust 48 for gas such as air or nitrogen, and a drain 50 for liquid.

In a typical process the semiconductor wafer 44 is secured to the chuck12 using any standard method such as a vacuum established between thechuck 12 and the wafer 44. A wafer transport door 52 to the housing 14is thereafter closed. The housing 14 is purged with dry solvent-freegas. Control gas is then fed into the housing. The solvent concentrationof the control gas can be controlled before, during and after thecoating solution is dispensed onto the substrate. By manipulating thevalves 38 and 40, the solvent is passed along the conduit 36 through thevalve 40, along the conduit 24, and into the housing 14. A controlledpartial pressure of solvent can be achieved by bubbling the gas,comprising nitrogen or air, through the bubbler 30. The bubbler 30 inthis embodiment includes a porous glass frit from which the gas ispassed through the liquid solvent 34 which is maintained at anappropriate set temperature. The resultant solvent-bearing gas,containing the appropriate concentration of solvent, is passed over thesemiconductor wafer prior to and during the coating process. Clearly thesolvent tank 32 must contain or be supplied with sufficient solvent toallow the desired solvent concentration in the solvent-bearing gas to bemaintained.

In order to deposit a layer of photoresist onto the wafer 44, thepolymer solution is applied across the surface of the wafer 44 via thedispensing head 42. This is achieved by dispensing the polymer solutionin a continuous stream from a nozzle 54 onto the wafer 44 while thewafer 44 is spinning at relatively low speed or while it is stationary.In the preferred embodiment, the nozzle 54 is moved substantiallyradially across the wafer 44. Instead, the solution can be dispensed atthe center of the substrate, or multiple nozzles can be used. Byadjusting the spin speed of the wafer 44, the movement of the nozzle 52and the rate at which the polymer solution is dispensed, a suitabledistribution of the solution can be achieved. In another embodiment, asillustrated in FIG. 2, the polymer solution is deposited onto the waferby means of a film extruder 60, which is a conventional extruder knownin the art, while the wafer is rotated by one full turn. The extruder60, deposits a film of polymer solution onto the wafer 70. If a wafernot having a substantially circular shape, is to be coated, the wafer istypically moved longitudinally during the polymer solution depositionstep.

Since the extruder process is otherwise identical to the FIG. 1embodiment, the process will be described with reference to FIG. 1.After the solution has been deposited on the wafer 44, the spin speed ofthe wafer 44 is increased to spread the solution across the uppersurface of the wafer 44. By exhausting the solvent-containing gas andany particulate contaminants suspended in the solvent-bearing gas viathe exhaust 48, before and during the coating of the wafer 44, a uniformlayer of photoresist pre-polymer solution can be formed on the uppersurface of the wafer 44. Thereafter the gas passed into the chamber viathe conduit 24 is switched to temperature and humidity controlledsolvent-free gas such as air or nitrogen by means of the valves 38, 40.The valves 38 and 40 are typically controlled by a microprocessor (notshown). The solvent concentration in the control gas is then decreasedor the temperature of the control gas increased to remove more of thesolvent in the polymer solution deposited on the wafer 44. Typically, asolvent-free gas is supplied to the housing to enhance the evaporationof the solvent from the polymer solution. When the desired amount ofevaporation has taken place to form a sufficiently hard layer ofphotoresist, the wafer 44 is stopped, the wafer transport door opened,and the coated wafer removed from the housing 14. As mentioned above,gas, which may be dry or humid, solvent-free or solvent-bearing gas, isremoved from the housing 1 4 during the various stages via the exhaust48. In this way the gas is channelled to pass over the wafer 44 from anupstream side at the shower-head dispenser 20 to a downstream side atthe exhaust 48. The exhaust flow is controllable by means of a valve 74,thereby allowing the gas pressure in the housing 44 to be controlled.The valve 74 is typically controlled by means of a microprocessor (notshown). Any polymer solution spun off, including particulatecontaminants, is collected in the annular channel 46 to be removed viathe drain 50.

As discussed above the temperatures of the liquid solvent or gassupplied to the bubbler 30 is adjustable. In this way the partialpressure of solvent in the solvent-bearing gas can be adjusted. This canbe achieved by making use of heating/cooling coils as described above.Instead the concentration can be adjusted by adding a gas bearing adifferent solvent concentration. This could be supplied via a conduit(not shown) connected to the conduit 36. Clearly if the temperatures ofthe bubbler and the housing 14 are the same, the solvent-bearing gaswill be saturated with solvent. If the temperature of the bubbler ishigher, the solvent-bearing gas in the housing 14 will be supersaturated, and if the temperature of the bubbler is lower, thesolvent-bearing gas in the housing 14 will be unsaturated. Typically thesolvent-bearing gas supplied by the bubbler 30, and the housing 14 willbe kept at the same temperature to maintain solvent saturation. Asmentioned above, the arrangement 10 is typically mounted in an outerhousing (not shown). The outer housing is temperature controlled tomaintain it and the components of the arrangement 10 at a temperature oftypically 22° C.

Usually the solvent contained in the bubbler and borne by the gas willbe the same as that contained in the solution deposited on the wafer.Similarly, if the solution contains more than one solvent, the bubblermay contain similar solvents in the same ratios. However it may bedesirable in certain circumstances to use different solvents in thebubbler as compared to the solvents in the solution deposited on thewafer.

It will be appreciated that a solvent-bearing gas can be produced usingtechniques other than a bubbler.

Solvent vapor pressure can be determined precisely by using anisoteniscope. Alternatively, by passing inert gas through a sample ofthe solution and measuring the amount of solvent removed as a functionof time gravimetrically, the vapor pressure can be determined precisely.The solvent partial pressure in the gas supplied by the conduit 36 canbe adjusted, optimally to correspond to the equilibrium vapor pressurecreated by the solvent in the polymer solution. This ensures that therate at which solvent evaporates from the deposited film or coating isequal to the rate at which the solvent is absorbed by the film from thegaseous environment.

As mentioned above the solvent partial pressure in the housing 14 can beadjusted by controlling the temperature of the bubbler or gas.Alternatively, a gas containing a different solvent concentration can bemixed with the solvent-saturated gas. The optimum profile of solventpartial pressure in the housing atmosphere as a function of time duringthe coating process can be determined empirically.

By continuously exhausting the solvent-bearing gas or humid air via theexhaust 48, humidity and solvent partial pressure in the housing canreadily be adjusted during the coating process to ensure a uniformsolution layer thickness on the semiconductor wafer surface. Similarlythe effects of premature evaporation of solvent from the polymersolution deposited on the wafer 44 can be eliminated. This allows lesspolymer solution to be used, thereby reducing costs.

Clearly the invention is not limited to the embodiments described above.The conduit 28 could, for instance, be connected directly to the conduit36. In this way the gas supplied by the temperature and humiditycontrolled source could also supply the bubbler 30. The humidity of thetemperature and humidity controlled source would simply be reduced tozero while supplying the bubbler 30. When dry or moist gas is to besupplied to the housing 14, a valve in the conduit 28 could be closed toinsure that no solvent is sucked up the conduit 28.

It should be noted that when certain advanced deep ultra-violetphotoresist materials are used, a moisture-free casting environment canbe used. Accordingly the humidity of the temperature and humiditycontrolled gas will be kept at zero.

It will further be appreciated that the invention is not limited in itsapplication to a spin coating arrangement but applies equally toarrangements making use of other solution deposition techniques.

The invention claimed is:
 1. A coating apparatus for coating a surfaceof a substrate with a polymer solution which includes an enclosedhousing; a rotatable chuck mounted in the housing for supporting thesubstrate; a depositing means for depositing the polymer solution ontothe surface of the substrate in the housing; a control gas supply meansconnected in flow communication with the housing for supplying a controlgas to the housing, the control gas supply means comprising a controlgas supply conduit communicating with the housing by means of an inlet;a solvent-bearing gas supply conduit and solvent-free gas supply conduitcommunicating with the control gas supply conduit; a gas control valvemounted in the solvent-bearing gas supply conduit and a gas controlvalve mounted in the solvent-free gas supply conduit for controlling therate of gas flowing from the solvent-bearing gas supply conduit and fromthe solvent-free gas supply conduit into the control gas conduit tocontrol the composition of solvent vapor in the control gas; and anexhaust means connected to the housing for exhausting the control gasand any solvent vapor and particulate contaminants from thehousing,whereby a desired concentration of solvent vapor in the controlgas can be provided to control the evaporation rate of solvent from thepolymer solution, the desired concentration of solvent vapor beingprovided by adjusting a gas control valve in at least one of thesolvent-bearing gas supply conduit and solvent-free gas supply conduitto provide a ratio of solvent-bearing gas and solvent-free gas whichgives said desired concentration.
 2. A coating apparatus of claim 1wherein at least one of said gas control valves is anelectrically-controlled valve.
 3. A coating apparatus of claim 1 whereinboth of said gas control valves are electrically-controlled valves.